In producing oil from subsurface reservoirs, generally a mixture of oil, water, gas and sand (solids) comes to the surface. It is essential that these components are separated into their single constituents. Separation of the gas phase and the solid phase is relatively easy, based on the relatively large differences in specific weight. The separation of two liquid phases is more difficult as the difference in specific weight are usually relatively low, while all kinds of mixtures may consist including stable dispersions. In the case of (crude) oil/water mixtures the differences in specific weight are often fairly small.
Traditionally oil/water separations are done in vessels where the separation is done by gravity. See for instance US 2003/0168391. Such separation vessels are large, heavy and maintenance sensitive. The separation process requires long residence times of the oil/water mixture in the vessel. Therefore, the inventory of such vessels is relatively large, which may create problems at the start-up, the closing down and any re-starts of such units.
There is a drive to achieve oil and water separations in devices that are more compact than the traditional gravity based equipment and in a faster way. In addition to the above there is also a need to make a bulk separation in which the major part of oil or water is separated from the mixture in a compact device followed by an additional step in which the separation in the single components is further improved.